Softening rubber



matter and to advance the art.

Registered Nov. 26, 1940 2r. BUBBEB Ira Williams, Berger, Tax, and Carroll 1 in; at?

Smith, deceased, late of ilarneys Point, N. 5., by Lelia Logan Smith, execntrix, Carneys Point, N. 3., assignors to E. I. du Pont ale Nemouis &

Company, Wilmington,

Delaware No Drawing.

11 Claims.

This invention relates to rubber and more par ticularly to methods of treating rubber to produce a more plastic product.

It is well known that rubber is rendered more plastic when it is subjected to mechanical working in the presence of oxygen. The degree of plastication is determined to a great extent by the temperature and the time of milling. This action can be greatly facilitated by the use of small amounts of certain assistants such as the mono-aryl-hydrazines and the thio-phenols. Such assistants, however, require time and/or high temperatures to produce their action. It

is frequently desirable to plasticize the rubber more rapidly and particularly during mechanical working.

It is an object of this invention to provide an improved method for plasticizing rubber. Another object is to provide a new class of compounds for plasticizing rubber. A further object is to provide a method for more rapidly plasticizing rubber and new materials which will produce such more rapid plasticizing. Still fur ther objects are to provide new compositions of Still other objects will appear hereinafter.

These objects may be accomplished in accordance with the present invention by subjecting rubber, during the mixing period, to the action of a small amount of a thio-carboxylic acid or its sulfide. The acids and their sulfides may be represented by the formula in which R is an organic radicle which may be aromatic, aliphatic or heterocyclic, and may be substituted by such groups as hydroxyl, allzoxy, carbonyl, nitro, halogen and mercapto groups, and X represents hydrogen or the group in which R represents an organic radicle which may be the same or different than R. R and R are each connected to the carbonyl carbon directly by means of a carbon to carbon linkage.

7 While the large class of thio-carboxylic acids and their sulfides appear to be effective for DcL, a coration oi Original application Gotober 8, 1937, Serial No. 167,982. Divided and plication January 23, 19%, r

our purposes, it will generally be preferred to employ the aromatic thio-carboxylic acids and their sulfides, and particularly those in which R and R. are aromatic hydrocarbons. By the term "an aromatic thio-carboxylic acid or its sulfide", we mean those in which the carboxyl carbon is directly bonded to a carbon of the aromatic ring. Of these aromatic compounds, those of the benzene series will generally be preferred.

The best results are obtained by the addition of the plasticizing agent directly to the rubber as early as possible in the mixing operation and in the absence of compounding ingredients. Sulfur has been iound to materially retard the action of the thio acids and any material amount of sulfur in excess of 3% almost completely prevents the action of small amounts of the thio acids. The efifect of sulfur may be overcome to a certain extent by the use of relatively large amounts of plasticizing agent. However, this is unnecessarily expensive and not setisfactory.

The salts oi the thio acids appear to be substantially inefiective. However, the salts of the thio acids may be employed in certain cases where the rubber contains sumcient acidic material, either naturally present or added, to release the thio acid so that it may exert its action. Accordingly, when we refer to the use of a thio acid hereinafter and in the claims, we intend to include :the salts when added in the presence of sufficient acidic material under conditions which will cause the acidic material to react with the salt to release the freethio acid or when other conditions are employed which will cause theliberation of the free acid from the salt.

Basic ingredients, such as zinc oxide and lime. exert a very marked retarding action and should be avoided. The basic materials which will seriously retard the action of our compounds are those which are sumciently active under the conditlons employed to react with the thio acids or sulfides to form. salts of the thio acids. When a thio acid is employed, sumcient basic material to completely neutralize the-acid and form a salt therewith will substantially completely prevent the action oi the acid.

Further, some accelerators and antioxidants 5o exert a retarding action on our compounds to various degrees and any substantial amounts of them in excess of 3% should be avoided during the plasticlzing period. In the case of the 5 basic accelerators .and antioxidants which are sufiiciently active to react with the thio acids or sulfides, thev rule as to basic materials will apply. Sulfur containing accelerators appear to act in a manner similar to free sulfur.

By the term sulfides," we intend to include the polysulfides, which readily lose the excess sulfur without imparting sufficient free sulfur to the rubber system to prevent the plasticizing action.

' Generally, our compounds will be employed in proportions of about 0.05 to about 2%, based on the rubber. Larger or smaller amounts may be employed if desired, but without advantage. Preferably, we employ from about .2% to about 1% of the plasticizing agent, based on the rub- .ber.

Our compoundswill be -eilective at room temperature. However, they will be more eifective at temperatures above about 50C, Preferably, temperatures of from about 100 C. to about 130 C. will be employed. In most cases, temperatures above 130 C. will not be required, although any temperature may be employed at which it is safe to heat rubber alone.

In order to more clearly illustrate our invention, the preferred modes of carrying the same into efiect and the advantageous results to be obtained thereby, the following examples are given. In these examples, the mixing was conducted-in a steam jacketed enclosed mixer. The plasticity of the rubber was determined with pendulum actuated parallel plate plastometer described in Industrial andEngineering Chemistry, Analytical Edition, 8 305 (1936). The plasticity number is proportional to thework required to deform the rubber, while the recovery is the thickness in millimeters recovered by the rubber after compression. The recovery figure appears to be or the most significance as indicating the degree of softness of the rubber. Y

Exluurml Fifty parts of smoked sheet rubber was milled for a total time of 10 minutes at 120 C., in each case. The rubber was placed in the; mixer and.

Table 1 8o Plasticity R my- Material added I Percent number an N 47 545 Thio-benzoic acid 0. 2 27 045 Do 0. 4 22 .005 Thio-anisic acid. 0. 5 2i 007 Benzoyl sulfide 0. 5 Z; 005 But 1 sulfide 0. 5 35 340 Furoy sulfide. 0. 2 34 280 Tbioeteario acld.. 0. 5 28 012 Thio-iurolc acid l. 0 25 .013 Z-hydrew-ii-thio-naphthoic acidl. 0 30 190 Anthraquinone 2 thio cerboxyllc acid 1. 0 42 450 Exnnm: 2

This example illustrates the retarding action of suliur and zinc oxide. The procedure of Example 1 was followed, but the zinc oxide and sulfur, when employed, were added one-half minute before the plasticizing agent. In the following Table II P denotes plasticity number, and R denotes. recovery.

Table II Other material added Agent added None 3% sulfur 5% sulfur 10% Zn!) PRPRPRPR' None 47 570 46 560 45 560 48 580 0.3% benzoyl disulfide. 29 085 38 500 41 530 43 480 0.2% zinc thio-benzoate 27 040 '40 .500 40 500 47 555 such as sodium sulfhydrate, sodium sulfide or other sodium sulfur compound. The methods of preparing the compounds are more clearly described in more detail in the literature.

Other thio-carboxylic acids which may be employed for our purpose in accordance with our invention are:

Tl'iio-acetic acid Thio-propionic acid Thio-butyric acid Thio-lsobutyric acid Thio-valeric acid Y Thio-isovaleric acid Thio-hexoic acid Thio-caprylic acid Thio-lauric acid Thio-myristic acid Thio-palmitic acid Thio-hydroxystearic acid Thio-acrylic acid Thio-crotonic acid Thio-oleic acid Thio-linolelc acid Thio-ricinoleic acid Thio-phenyl acetic acid Thio-cinnamic acid Thio-hydrocinnamic acid Thio-phenyl acrylic acid Thio-naphthenic acids Thlo-toluic acids Thlo-xyloic acids Mesitylene thio-carboxylic acids Thio-ethylbenzoic' acids Mono-thio-phthaiic acids l Iydroxy-methyl-thiobenzoic acids Thlo-chlorbenzoie acids Thio-hexahydrobenzoic. acids 'I'hio-a-naphthoic acids 'I'hio-beta-naphthoic acids Thio-anthranoic acids Thio-phenylbenzoic acids Other sulfides which may be employed are:

Toluyl sulfide Toluyl disulfide Xyloyl sulfide xyloyidisulnde aaaaoes Mesityloyl sulfide Mesityloyl dlsulfide Ethylbenzoyl sulfide Ethylbenzoyl disulflde o-Carboxyl-berizoyl sulfide o-Carboxyl-benzoyl disulfide Hydroxy-methyl benzoyl sulfide Hydroxy-methyl benzoyl dlsulfide Chlorbenzoyl sulfide Chlorbenzoyl disulfide Hexahydrobenzoyl sulfide Hexahydrobenzoyl disulfide a-Naphthoyl sulfide a-Naphthoyl disulfide b-Naphthoyl sulfide b-Naphthoyl disulfide Anisoyl sulfide Anisoyl disulfide Anthranoyl sulfide Anthranoyl disulfide Phenylbenzoyl sulfide -Phenylbenzoyl disulfide Acetyl sulfide Acetyl disulfide Propionyl sulfide Isobutyryl sulfide Isobutyryl disulfide Valeryl sulfide Caproyl sulfide Caproyl disulfide Lauroyl sulfide Lauroyl disulfide stearoyl sulfide Stearoyl disulfide Hydroxystearoyl sulfide Hydroxystearoyl disulfide Acrylyl sulfide Acrylyl disulfide Oleyoyl sulfide Oleyoyl disulfide Linoleoyl sulfide Linoleoyl disulfide Phenylacetyl sulfide Phenylacetyl disulfide Cinnamyl sulfide Cinnamyl disulfide Hydrocinnamyl sulfide Hydrocinnanrvl disulfide Phenylacrylyl sulfide Phenylacrylyl 'disulfide Naphthenoyl sulfide Naphthenoyl disulfide 2-hydroxy-3-naphthoyi sulfide 2-hydroxy-3-naphthoyi disulfide Sulfide of anthraqulnone-2-thio carboxylic acid I Disulfide of anthraqulnone-2-thio-carboxylic acid - The various homologues and isomers or the compounds mentioned will also be efiective. The radicle R in the formula may be varied within wide limitasince the ac-= tivity of the thio acid group is so great that the introduction of such substituents as nitroor plasticizins agent may be added to the rubber before solution in the solvent. The preparation of cements of low viscosity may be most advantageously efiected by dissolving rubber in a solvent containing the plasticizing agent. The plasticizing agents may also be incorporated in the rubber by addition to latex before coagulation.

While we have disclosed the incorporation of our plasticizing agents with rubber in a heated internal mixer, other types of apparatus may be employed, such as an ordinary rubber mill, Banbury type of mixer or a Gordon type plasticator. These apparatus will also operate more efliciently at the elevated temperatures.

This is a division of our copending application, Serial No. 167,982, for softening-rubber, filed October 8, 1937.

While we have disclosed the preferred embodiments oi. our invention and the preferred modes of carrying the same into efifect, it will be readily apparent to those skilled in the art that many changes and variations may be made therein without departing from the spirit of our invention. Accordingly, the scope of our'invention is to be limited solely by the appended claims,

construed as broadly as is permissible in view of the prior art. a

We claim:

1. Rubber obtained by subjecting unvulcanized rubber to the action of a thio-carboxylic acid compound having the formula wherein R and R represent organic groups connected to the carboxylic carbons directly by carbon to carbon linkages, in the absence of more than about 3% of sulfur and in the absence of sumcient basic material to neutralize the action of the thio-carbozwlic acid compound, for a suificient length of time for said thio-carboxylic acid compound to effect. a marked decrease in the resistance of the rubber to flow over that which the rubber would have if subjected to the same conditions in the absence of said thiccarboxylic acid compound.

2. Rubber obtained by subjecting unvuicanized rubber to the action of a thio-carboxylic acid compound having the formula 0 o R- Sr-( R wherein R and R represent aromatic radicles connected to the carboxylic carbons directly by carbon to carbon linkages, in the absence of more than about 3% 01 sulfur and in the absence of sufiicient basic material toneutralize the action of the thio-carboxylic acid compound, for a sufficient length of time for said thio-carboxylic acid compound to effect a marked decrease in the resistance of the rubber to flow over that which the rubber would have if subjected to the same conditions in the absence. of said thio-carboxylic acid compound.

3. Rubber obtained by subjecting unvulcanized rubber to the action ofa thio-carboxylic acid compound having the formula 1 wherein R and R. represent hydrocarbon radicles, in the absence of more than about 3% of sulfur and in the absence of sumcient basic material to neutralize the action or the thio-carboxylic acid compound, for a sufiicient length 0! time for said thio-carboxylic acid compound to enact a marked decrease in the resistance of the rubber to flow over that which the rubber would have'if subjected to the same conditions in the absence of said thio-carboxylic acid compound.

4. Rubber obtained by subjecting unvulcanized rubber to the actionof a thio-carboxylic acid compound having the formula wherein R. and R represent aromatic hydrocarbon radicles, in the absence of more than about 3% of sulfur and in the absence of sufilclent basic material to neutralize the action of the thio-carboxylic acid compound, for a sufficient length of time for said thio-carboxylic acid compound to eifect a marked decrease in the resistance of the rubber to flow over that which the rubber would have if subjected to the same conditions in the absence of said thio-carboxylic acid compound.

5. Rubber obtained by subjecting unvulcanized rubber to the action of a thio-carboxylic acid compound having the formula wherein R and R represent aromatic radicles of the benzene series connected to the carboxylic carbons directly by carbon to carbon linkages, in the absence of more than about 3% of sulfur and in the absence of sufl icient basic material to neutralize the action of the thio-carboxylic acid compound, for a suflicient length of time for said I thio-carboxylic acid compound to effect a marked decrease in the resistance of the rubber to flow over that which the rubber would have if subjected to the same conditions in the absence of said thio-carboxylic acid compound.

- 6. Rubber obtained by subjecting unvulcanized rubber to the action of a thio-carboxylic acid boxylic acid compound to effect a markeddecrease in the resistance of the rubber to flow over that which the rubber would have if subjected to the same conditions in the absence of said thiocarboxylic acid compound.

7. Rubber obtained by subjecting unvulcanized rubber to the action of benzoyl disulflde, in the absence of more than about 3% of sulfur and in the absence of sufficient basic material to neutralize the action of the benzoyl disulflde, for a suflicient length of time for said benzoyl disulfide to efiect a marked decrease in the resistance of the rubber to flow over that which the rubber would have if subjected to the same conditions in Y the absence of said benzoyl disulfide.

8. The method of decreasing the resistance to how of unvulcanized rubber which comprises subjecting the unvulcanized rubber to the action aaeaoes of a thiocarboxylic acid compound having the formula wherein R and R represent organic groups conv nected to the carboiwlic carbons directly by carbon to carbon linkages, in the absence of more than about 3% of sulfur and in the absence of sufficient basic material to neutralize the action of the thio-carboxylic acid compound, for a sumcient length oftime for said thio-carboxylic acid compound to effect a marked decrease in the resistance of the rubber to flow over that which the rubber would have if subjected to the same conditions in the absence ofsaid thio-carboxylic acid compound.

9. The method of decreasing the resistance to flow of unvulcanized rubber which comprises subjecting the unvulcanized rubber to the action of a thio-carboxylic acid compound having the formula R-lL-SrE-R' wherein R and R'- represent hydrocarbon radicles, in the absence of more than about 3% of sulfur and in the absence of sufiiclent basic material to neutralize the action of they thio-carboxylic acid compound, for a suflicient length of time for said thio-carboxylic acid compound to effect a marked decrease in the resistance of V the rubber to flow over that which the rubber would have if subjected to the same conditions in the absence of said thio-carboxylic acid compound. g

10. The method of decreasing the resistance to ,flow of unvulcanized rubber which comprises subjecting the unvulcanized rubber to the action of a thio- -carbcxylic acid compound having the formula a o o Ri-a-t-R' wherein R and R represent aromatic hydrocar- I to flow of unvulcanized rubber which comprises subjecting the unvulcanized rubber to the action of benzoyl disulfide, in the absence of more than about 3% of sulfur and in the absence of suiiicient basic material to neutralize the action of the benzoyl disulfide for a sufllch at length of time for said benzoyl disulflde to effect a marked decrease in the resistance of the rubber to flow over that which the rubber would have if subjected to the same conditions in the absence of said benzoyl disulfide.

IRA WILLIAMS. LELIA LOGAN SMITH, Executria: of the Estate of Carroll Cummings Smith, Deceased. 

